Crustal Azimuthal Anisotropy Research Articles

Crustal Azimuthal Anisotropy in Eastern North China Derived from Ambient Noise Tomography

Crustal Azimuthal Anisotropy in Eastern North China Derived from Ambient Noise Tomography

An iterative weighted least-square fitting method for crustal anisotropy using receiver functions

SUMMARY The harmonic variation of the P-to-S converted phases (i.e. Pms) observed from receiver functions (RFs) includes information on crustal azimuthal anisotropy. However, this harmonic analysis is easily influenced by low-quality RF traces, and the measurements may be misleading. Here, we propose an improved method, named the iterative weighted least-square method (IWLS), to extract the splitting parameters of the crust and simultaneously retrieve the two- and four-lobed components of backazimuthal variation. The quality and weights of different RF traces are estimated properly in the IWLS method. The weight function is related to the sharpness of the Pms phase and the smearing of other signals. We conduct many synthetic tests, and the IWLS method provides stable measurements for poor backazimuthal coverage, strong noise, weak P-wave azimuthal anisotropy and multiple anisotropic layers. We apply the IWLS method to observational data from two temporary stations on the southeastern Tibetan Plateau and North China Craton, respectively. The measurements are comparable to previous results and provide insight into crustal deformation.

Azimuthal Anisotropy of Receiver Functions in the Central South China Block and its Tectonic Implications

By the H-к stacking of the receiver functions and the splitting of the Pms phases, using seismic data from the Regional Seismic Network and the Huanan Seismic Array, a high-resolution temporary seismic array deployed for 2 years in the study area. This study revealed the strong lateral heterogeneity in crustal structures in the central South China block. Crustal thickness reduces from northwest to southeast, with significant differences across the boundary of sub-blocks. The average crustal Vp/Vs ratio gradually increases from west to east, leading to high values in the coastal region, which suggests that the subduction of the Pacific plate has possibly caused the underplating of magma or the upwelling of upper mantle material. The crustal azimuthal anisotropy of the Dabie orogen and the Jiangnan orogen is generally consistent with the strike of the tectonic belt as well as with the orientation of the absolute plate movement. We suggest that the crustal azimuthal anisotropy of the orogen is related to the extension and deformation of the lithosphere. The anisotropy in the crust is close related to crustal deformation. The orientation in the crust and the upper mantle in the Cathaysia block are generally consistent with the orientation of the absolute plate motion, indicating that the azimuthal anisotropy of the Cathaysia block is related to lithospheric deformation and the under-invasion of upper mantle material.

Crustal azimuthal anisotropy in the lateral collision zone of the SE margin of the Tibetan Plateau and its tectonic implications

SUMMARYResolving the detail of crustal deformation of the seismically and technically active Sichuan-Yunnan block located in the southeast margin of the Tibetan Plateau is crucial for understanding the lateral extrusion of the Tibetan Plateau. Using seismic data from two regional seismograph stations of the China Earthquake Networks Center and twenty recently deployed broad-band temporary stations from the MCD Array (Middle ChuanDian block seismic array), we constrained crustal thickness and seismic anisotropy at the west boundary of the Sichuan-Yunnan block, near the east Himalayan syntaxis by receiver function techniques including CCP stacking and sinusoidal moveout of PmS conversions. Crustal thickness varies significantly across major faults inside the block which increases by >20 km from west to east. East of the Chenghai Fault, a clear positive phase lies above the Moho and is interpreted to be caused by the magma intrusion related to a late Permian Emeishan large igneous province (ELIP). The crustal anisotropy shows strong zoning features and clockwise rotation with respect to due north. To the west of the Lijiang-Xiaojinhe and the Honghe Faults, anisotropic fast orientations are near NS, indicating strong crustal deformation and movement of southward crustal flow along the Nujiang-Lancangjiang Fault; to the east of this fault, anisotropic fast orientations are near NE. The variation of the anisotropic fast orientations indicates that the southeastward crustal flow from the Tibetan Plateau may be prevented by fast velocity anomalies beneath the ELIP. These observations provided new evidence for strong crustal deformation along the tectonic boundary in the southeastern Tibetan Plateau with the expansion of the Tibetan Plateau.

Depth-dependent crustal azimuthal anisotropy in the Salton Trough, southern California

We present new crustal models of azimuthally anisotropic P-wave velocity, isotropic P-wave velocity (Vp), isotropic S-wave velocity (Vs) and Vp/Vs ratio for the Salton Trough. High Vp/Vs ratio is revealed at depths of 2–8 km along the San Andreas Fault, implying the possible presence of fluids. High Vp and high Vp/Vs ratio (>1.8) structures of the mid-lower crust beneath the Salton Trough possibly reflect the underplated gabbroic rocks as a result of the extension-induced partial melting of the upwelling asthenospheric materials. The fast velocity directions (FVDs) generally correlate with the direction of the maximum horizontal compressive stress except that fault-parallel FVDs are found at main fault traces. However, the FVDs in the Salton Trough show complex features. The northern Slaton Trough basin (the northern Salton Sea basin and Coachella Valley basin) is dominated by NW-SE FVDs, which are possibly due to the active compression happening there. E-W oriented FVDs predominate the upper crust of the southern Salton Trough basin (the southern Salton Sea basin and Imperial Valley basin), possibly due to faulting and/or block rotation. As the depth increases (12–18 km), the FVDs to the south of the Salton Sea have an overall rotation and gradually become NW-SE. The depth-dependent variation of anisotropy orientation may be attributed to a large band of underlying heat source that probably causes a change of stress field by imposing thermal stress on overlying rocks. In all, our velocity models suggest that faults in the Salton Trough may bear a high risk of mechanical failures due to the complexities of stress distribution and geological structures as well as the presence of a large volume of fluids and heat.

3‐D Crustal Azimuthal Anisotropy Reveals Multi‐Stage Deformation Processes of the Sichuan Basin and Its Adjacent Area, SW China

AbstractThe collision of the South China Block and the North China Block and the eastward extrusion of the Tibetan Plateau materials have caused complicated tectonic deformation and strong seismicity around the Sichuan Basin (SCB). In order to reveal regional stress distribution and deformation mechanisms, we invert for a 3‐D high‐resolution shear wave velocity and azimuthal anisotropy model from ambient noise tomography with data from 483 broadband seismographs deployed around the SCB. Our results reveal strong lateral contrasts of azimuthal anisotropy in the crust. The western Sichuan depression shows weak deformation. The central Sichuan uplift, used to be a stable microcontinent, shows relatively strong and coherent fossil anisotropy. In contrast, the eastern Sichuan folds shows strong anisotropy, indicating intense regional deformation. The fast axis direction is bifurcated when reaching the Longmenshan fault, which indicates the obstruction of the SCB to the crustal low velocity materials beneath the Songpan‐Ganzi Block. In the east, the rigid SCB and strike‐slip faults may transform the compressive stress to shear deformation in adjacent areas and cause a ring‐shaped anisotropy pattern below 15 km depth. Furthermore, anisotropy patterns display strong contrast at shallow depths in the Weiyuan and Changning regions, which may facilitate the accumulation of strain and more likely induce earthquakes during the shale gas exploration stage. Our new model provides important constraints for understanding the multi‐stage deformation processes in and around the SCB.

Crustal azimuthal anisotropy and deformation beneath the northeastern Tibetan Plateau and adjacent areas: Insights from receiver function analysis

We present crustal azimuthal anisotropy measurements at 246 stations by fitting the sinusoidal moveout of P-to-S receiver functions to infer crustal deformation field in the northeastern (NE) Tibetan Plateau and adjacent areas. The dominant orientation of the observed crustal anisotropy shows a systematic spatial variation: NW-SE in the NE Tibetan Plateau and western Alxa Block, nearly NE-SW in the eastern Alxa Block and Yinchuan-Hetao Graben, and E-W in the eastern portion of the Western Qinling Terrane. The magnitude of crustal anisotropy varies from 0.15 to 0.75 s with an average of 0.36 ± 0.11 s. Fault parallel crustal anisotropy with large magnitudes observed in the NE Tibetan Plateau provides independent constraints on the lateral extent of the deformation zone associated with the faults. The observed spatial distribution of crustal anisotropy, when combined with previous crustal velocity and Poisson's ratio measurements, implies that channel flow in the middle-to-lower crust is unlikely to exist beneath the NE Tibetan Plateau. The main characteristics of crustal anisotropy observed in the western Alxa Block are similar to areas affected by the Indo-Eurasian collision, while those observed in the eastern Alxa Block are comparable to the stable North China Craton.

Crustal azimuthal anisotropy in the Jiaodong Peninsula: Evidence for the suture between the North China Craton and South China Block

The Jiaodong Peninsula is an ideal region for studying the geodynamics of collision between the North China Craton (NCC) and South China Block (SCB). It is comprised of the Jiaobei massif and the Northern Sulu UHP massif which are separated by the Wulian suture zone (WSZ). Continent-continent collision is necessarily accompanied with crustal deformation. Consequently a detailed crustal deformation pattern would offer observational constraints for understanding the process of the collision. In this study, we measured the crustal azimuthal anisotropy beneath a broadband seismic profile of 20 stations together with 9 permanent stations in the Jiaodong Peninsula. Sinusoidal moveout of Pms phases from radial receiver functions and azimuth-weighted stacking of transverse receiver functions were jointly applied to ensure the reliability of results. In the WSZ, the fast directions are roughly parallel with the NE oriented fault strike, suggesting the Jiaodong Peninsula may preserve fossilized anisotropy in the crust induced by the Triassic collision of the NCC and SCB although have undergone subsequent lithosphere destruction and thinning since the late Mesozoic. We therefore support that the WSZ represents the NCC-SCB suture east of Tanlu fault. In the Jiaobei massif, the crustal azimuthal anisotropy is controlled by both the nearly E-W local stress and NNE-strike detachment faults. The anisotropy beneath the Northern Sulu massif is quite weak with delay times all less than 0.20 s, probably related to the high-density UHP metamorphic rocks which are difficult to generate strong anisotropy inside.

Layered crustal azimuthal anisotropy beneath the northeastern Tibetan Plateau revealed by Rayleigh-wave Eikonal tomography

The uplift of the Tibetan Plateau in the Cenozoic has greatly affected the tectonics in Asia. But the evolution of the plateau is still unclear. The northeastern Tibetan Plateau is under ongoing expansion and is therefore essential for understanding the tectonic evolution of the plateau. Using data from 675 stations deployed by the ChinArray project, we obtained an anisotropic shear-wave velocity model by Rayleigh-wave phase-velocity Eikonal tomography and further investigated layered anisotropy and deformation patterns in the northeastern Tibetan Plateau. In the upper crust (<20 km depth), the fast directions are parallel with major strike-slip faults. In the lower crust, the fast directions are mostly margin-parallel along the plateau margin (at depths of ∼20 km to Moho), but they are perpendicular to the plateau margin while parallel to the topographic gradient under the high plateau (at depths of ∼40 km to Moho). Our result indicates that there is lower crustal flow under the high plateau due to the differential lateral gravity potential. However, the deep crustal flow is blocked by the surrounding blocks, which results in pure shear along the plateau margin in the northeastern Tibetan Plateau.

Azimuthal Anisotropy of the Crust and Uppermost Mantle Beneath Alaska

AbstractThis study presents an azimuthally anisotropic shear wave velocity model of the crust and uppermost mantle beneath Alaska, based on Rayleigh wave phase speed observations from 10 to 80 s period recorded at more than 500 broadband stations. We test the hypothesis that a model composed of two homogeneous layers of anisotropy can explain these measurements. This “Two‐Layer Model” confines azimuthal anisotropy to the brittle upper crust along with the uppermost mantle from the Moho to 200 km depth. This model passes the hypothesis test for most of the region of study, from which we draw two conclusions. (a) The data are consistent with crustal azimuthal anisotropy being dominantly controlled by deformationally aligned cracks and fractures in the upper crust undergoing brittle deformation. (b) The data are also consistent with the uppermost mantle beneath Alaska and surroundings experiencing vertically coherent deformation. The model resolves several prominent features. (1) In the upper crust, fast directions are principally aligned with the orientation of major faults. (2) In the upper mantle, fast directions are aligned with the compressional direction in compressional tectonic domains and with the tensional direction in tensional domains. (3) The mantle fast directions located near the Alaska‐Aleutian subduction zone and the surrounding back‐arc area form a toroidal pattern that is consistent with mantle flow directions predicted by recent geodynamical models. Finally, the mantle anisotropy is remarkably consistent with SKS fast directions, but to fit SKS split times, anisotropy must extend below 200 km depth across most of the study region.

A Method for Estimating the Crustal Azimuthal Anisotropy and Moho Orientation Simultaneously Using Receiver Functions

AbstractThe P‐to‐S converted phase at the Moho (the Pms phase) contains important information regarding the crustal structure. Here, we employ a grid search approach to simultaneously extract the bulk crustal azimuthal anisotropy and Moho orientation using receiver functions. The solution of this approach is the weighted average of two individual subfunctions that separately analyze the birefringence and arrival time of the Pms phase. Since a tilted Moho deflects the polarized components of the Pms phase, the Pms birefringence subfunction minimizes the energy on all the transverse polarization components (i.e., the transverse polarization direction before Pms splitting). The implementation of this subfunction is similar to that of the transverse component minimization method, but the Moho orientation is considered. The Pms arrival time subfunction takes into account both the crustal anisotropy and the Moho orientation to predict and fit Pms arrivals. According to extensive synthetic experiments, this approach is remarkably robust to the presence of strong anisotropy, a steep Moho, and slightly tilted fast axes (less than 30°). This approach is also highly tolerant of random noise and data with uneven azimuthal coverage. We apply this approach to three permanent stations on the Tibetan Plateau. The resulting measurements boast a high quality and are consistent with the crustal anisotropy and Moho orientation revealed in previous studies.

Direct Inversion for Three‐Dimensional Shear Wave Speed Azimuthal Anisotropy Based on Surface Wave Ray Tracing: Methodology and Application to Yunnan, Southwest China

AbstractAzimuthal anisotropy retrieved from surface waves is important for constraining depth‐varying deformation patterns in the crust and upper mantle. We present a direct inversion technique for the three‐dimensional shear wave speed azimuthal anisotropy based on mixed‐path surface wave traveltime data. This new method includes two steps: (1) inversion for the 3‐D isotropic Vsv model directly from Rayleigh wave traveltimes and (2) joint inversion for both 3‐D Vsv azimuthal anisotropy and additional 3‐D isotropic Vsv perturbation. The joint inversion can significantly mitigate the trade‐off between strong heterogeneity and anisotropy. With frequency‐dependent ray tracing based on 2‐D isotropic phase speed maps, the new method takes into account the ray‐bending effect on surface wave propagation. We apply the new method to a regional array in Yunnan, southwestern China. Using Rayleigh wave phase velocity dispersion data in the period band of 5–40 s extracted from ambient noise interferometry, we obtain a 3‐D model of shear wave speed and azimuthal anisotropy in the crust and uppermost mantle in Yunnan. This model reveals that two midcrust low‐velocity zones are possible weak channels, and the azimuthal anisotropy at a depth of 5 to 30 km is mainly controlled by nearby strike‐slip faults, some of which also approximately coincide with the lateral boundaries of the crustal low‐velocity zones. Approximately south of 26°N, the upper crustal azimuthal anisotropy from our model is significantly different from the upper mantle anisotropy inferred by shear wave splitting, indicating different deformation styles between the crust and upper mantle in southern Yunnan.

Crustal Azimuthal Anisotropy Beneath the Central North China Craton Revealed by Receiver Functions

AbstractTo characterize crustal anisotropy beneath the central North China Craton (CNCC), we apply a recently developed deconvolution approach to effectively remove near‐surface reverberations in the receiver functions recorded at 200 broadband seismic stations and subsequently determine the fast orientation and the magnitude of crustal azimuthal anisotropy by fitting the sinusoidal moveout of the P to S converted phases from the Moho and intracrustal discontinuities. The magnitude of crustal anisotropy is found to range from 0.06 s to 0.54 s, with an average of 0.25 ± 0.08 s. Fault‐parallel anisotropy in the seismically active Zhangjiakou‐Penglai Fault Zone is significant and could be related to fluid‐filled fractures. Historical strong earthquakes mainly occurred in the fault zone segments with significant crustal anisotropy, suggesting that the measured crustal anisotropy is closely related to the degree of crustal deformation. The observed spatial distribution of crustal anisotropy suggests that the northwestern terminus of the fault zone probably ends at about 114°E. Also observed is a sharp contrast in the fast orientations between the western and eastern Yanshan Uplifts separated by the North‐South Gravity Lineament. The NW‐SE trending anisotropy in the western Yanshan Uplift is attributable to “fossil” crustal anisotropy due to lithospheric extension of the CNCC, while extensional fluid‐saturated microcracks induced by regional compressive stress are responsible for the observed ENE‐WSW trending anisotropy in the eastern Yanshan Uplift. Comparison of crustal anisotropy measurements and previously determined upper mantle anisotropy implies that the degree of crust‐mantle coupling in the CNCC varies spatially.

A method of simultaneously measuring crustal azimuthal anisotropy and Moho geometry based on receiver functions

A method of simultaneously measuring crustal azimuthal anisotropy and Moho geometry based on receiver functions

Crustal Azimuthal Anisotropy Beneath the Southeastern Tibetan Plateau and its Geodynamic Implications

AbstractThe fast orientation and magnitude of crustal azimuthal anisotropy beneath the southeastern Tibetan Plateau and adjacent areas are measured by analyzing the sinusoidal moveout of the P to S converted phase from the Moho. Beneath the tectonically active plateau, the mean magnitude is 0.48 ± 0.13 s, which is about twice as large as that observed in the stable Sichuan Basin (0.23 ± 0.10 s). The two areas are separated by the Longmenshan fault zone, a zone of devastating earthquakes including the 12 May 2008 MW 7.9 Wenchuan earthquake. Fault orthogonal fast orientations observed in the southern Longmenshan fault zone, where previous studies have revealed high crustal Vp/Vs and suggested the presence of mid‐lower crustal flow, may reflect flow‐induced lattice preferred orientation of anisotropic minerals. Fault parallel anisotropy in the central segment of the fault zone is most likely related to fluid filled fractures, and fault perpendicular extensional cracks are probably responsible for the observed anisotropy in the northern segment. The crustal anisotropy measurements, when combined with results from previous studies, suggest the existence of mid‐lower crustal flow beneath the southeastern margin of the plateau. Comparison of crustal anisotropy obtained before and after the Wenchuan earthquake suggests that the earthquake has limited influence on whole crustal anisotropy, although temporal changes of anisotropy associated with the earthquake have been reported using splitting of shear waves from local earthquakes occurred in the upper crust.

Crustal azimuthal anisotropy in the trans-North China orogen and adjacent regions from receiver functions

The North China Craton (NCC) is an important part of eastern China. Recent studies have shown that the eastern NCC (ENCC) has undergone significant lithospheric thinning and destruction since the late Mesozoic. Destruction of the cratonic lithosphere is necessarily accompanied by crustal deformation. Therefore, a detailed crustal deformation model can provide basic observational constraints for understanding the process and mechanisms of the destruction of the NCC. In this study, we estimated the crustal azimuthal anisotropy beneath 198 broadband stations in the NCC with a joint analysis of Ps waves converted at the Moho from radial and transverse receiver function data. We also performed a harmonic analysis to test the reliability of the measured anisotropy. We obtained robust crustal azimuthal anisotropy beneath 23 stations that are mostly located on the western margin of the Bohai Bay Basin, Yin-Yan orogenic belt, and Taihang Mountains, which reflects the crustal deformation characteristics in those regions. The crustal shear wave splitting time was found to range from 0.05 s to 0.68 s, with an average value of 0.23 s, which reveals a distinct crustal anisotropy in the Trans-North China Orogen (TNCO) and its adjacent areas. Our analysis of the results suggests that the strong NW-SE tectonic extension in the late Mesozoic and Cenozoic played an important role in crustal anisotropy in this region. In addition, the E-W trending crustal anisotropy on the margin of the Bohai Bay Basin indicates an effect of the ENE-WSW trending horizontal principal compressive stress. The crustal anisotropy in the Yin-Yan orogenic belt may be an imprint of the multiple-phase shortening of a dominant N-S direction from the early-to-middle Jurassic to the Early Cretaceous. Stations in the Taihang Mountains show large splitting times and well-aligned NW-SE fast directions that correlate with those measured from SKS splitting and that are possibly related to the lithospheric modification and magmatic underplating from the Late Mesozoic to Cenozoic in this area.

Crust-mantle coupling mechanism in Cameroon, West Africa, revealed by 3D S-wave velocity and azimuthal anisotropy

To understand the depth variation of deformation beneath Cameroon, West Africa, we developed a new 3D model of S-wave isotropic velocity and azimuthal anisotropy from joint analysis of ambient seismic noise and earthquake surface wave dispersion. We found that the Cameroon Volcanic Line (CVL) is well delineated by slow phase velocities in contrast with the neighboring Congo Craton, in agreement with previous studies. Apart from the Congo Craton and the Oubanguides Belt, the uppermost mantle revealed a relatively slow velocity indicating a thinned or thermally altered lithosphere. The direction of fast axis in the upper crust is mostly NE-SW, but trending approximately N-S around Mt. Oku and the southern CVL. The observed crustal azimuthal anisotropy is attributed to alignment of cracks and crustal deformation related to magmatic activities. A widespread zone of weak-to-zero azimuthal anisotropy in the mid-lower crust shows evidence for vertical mantle flow or isotropic mid-lower crust. In the uppermost mantle, the fast axis direction changed from NE-SW to NW-SE around Mt. Oku and northern Cameroon. This suggests a layered mechanism of deformation and revealed that the mantle lithosphere has been deformed. NE-SW fast azimuths are observed beneath the Congo Craton and are consistent with the absolute motion of the African plate, suggesting a mantle origin for the observed azimuthal anisotropy. Our tomographically derived fast directions are consistent with the local SKS splitting results in some locations and depths, enabling us to constrain the origin of the observed splitting. The different feature of azimuthal anisotropy in the upper crust and the uppermost mantle implies decoupling between deformation of crust and mantle in Cameroon.

Lateral variation of crustal properties from aerogeophysical data in northern Brazil

A new method is proposed for the study of crustal azimuthal anisotropy, and the lateral variation of other crustal aerogeophysical parameters, using a quantitative approach. The processing of a large, newly acquired, high-resolution set of aerogeophysical data is considered using the existing Radon transform and the geostatistical analysis approaches. The data set includes an area of the Guyana shield, in northern Brazil, which was not included in previous surveys. The area is covered by dense rain forest vegetation and thick soil layers. Parameterization was performed considering the possible anisotropic character of the geophysical 2D data. Application of the newly proposed geostatistical data processing yielded high-resolution images of the lateral variation of quantitative geophysical parameters, which indicate good correlation with previously determined seismic anisotropy in the area. Average anisotropy as measured by the Radon transform and variogram analysis is scale dependent. At scales greater than [Formula: see text], the results seem to match those of the previous seismic studies. Images of the derived quantitative parameters from magnetic and radiometric data in the shield area indicate sharp changes that follow the known geologic changes observable in the survey area. Observed large-scale anisotropy in the area seems to be autocorrelated with two different geochronological provinces of the Amazonian craton. Application of the new technique to two other lower resolution aeromagnetic surveys located on the Amazon basin has produced two different results, including one successful data treatment of the aeromagnetic survey. The results obtained for this area indicate that the sedimentary cover seems to play almost no role in the observed magnetic anomaly anisotropy in the middle Amazon basin area. Most of the observed magnetic anomalies appear to be controlled by the continuing geologic features of the cratonic basement.

Upper crustal azimuthal anisotropy across the contiguous U.S. determined by Rayleigh wave ellipticity

AbstractConstraints on upper crustal seismic anisotropy provide insight into the local stress orientation and structural fabric, but such constraints are scarce except in areas with dense recordings of local seismicity. We investigate directionally dependent Rayleigh wave ellipticity, or Rayleigh wave H/V (horizontal to vertical) amplitude ratios, between 8 and 20 s period across USArray to infer azimuthal anisotropy in the upper crust across the contiguous U.S. To determine the H/V ratios, we use all available multicomponent ambient noise cross correlations between all USArray stations operating between 2007 and 2013. In many locations, the observed H/V ratios are clearly back azimuth dependent with a 180° periodicity, which allows the fast directions and amplitudes of upper crustal anisotropy to be determined. The observed patterns of anisotropy correlate well with both near‐surface geological features (e.g., the Intermountain Seismic Belt and Appalachian‐Ouachita collision belt) and a previous stress model.

Heterogeneity and anisotropy of the lithosphere of SE Tibet from surface wave array tomography

Understanding the geotectonic evolution of the southeastern Tibetan plateau requires knowledge about the structure of the lithosphere. Using data from 77 broadband stations in SW China, we invert Rayleigh wave phase velocity dispersion curves from ambient noise interferometry (T = 10–40 s) and teleseismic surface waves (T = 20–150 s) for 3‐D heterogeneity and azimuthal anisotropy in the lithosphere to ∼150 km depth. Our surface wave array tomography reveals (1) deep crustal zones of anomalously low shear wave speed and (2) substantial variations with depth of the pattern of azimuthal anisotropy. Upper crustal azimuthal anisotropy reveals a curvilinear pattern around the eastern Himalayan syntaxis, with fast directions generally parallel to the main strike slip faults. The mantle pattern of azimuthal anisotropy is different from that in the crust and varies from north to south. The tomographically inferred 3‐D variation in azimuthal anisotropy helps constrain the source region of shear wave splitting. South of ∼26°N (off the high plateau) most of the observed splitting can be accounted for by upper mantle anisotropy, but for stations on the plateau proper (with thick crust) crustal anisotropy cannot be ignored. On long wavelengths, the pattern of azimuthal anisotropy in the crust differs from that in the mantle. This is easiest explained if deformation varies with depth. The deep crustal zones of low shear wave speed (and, presumably, mechanical strength) may represent loci of ductile deformation. But their lateral variation suggests that in SE Tibet (localized) crustal channel flow and motion along the major strike slip faults are both important.